Catalyst components for the polymerization of olefins include Ziegler-Natta category components. In a first instance, a Ziegler-Natta catalyst system was based on the use of solid TiCl3 obtained by reduction of TiCl4 with aluminum alkyls. Because of the insufficient activity and stereospecificity of the catalysts, the resulting polymer were subjected to a de-ashing treatment to remove the catalyst residues and a washing step to remove any atactic polymer. Presently and in some instances, the Ziegler-Natta catalysts that are used industrially are made from or contain a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and optionally an internal electron donor compound, used in combination with an Al-alkyl compound.
In some instances, when ZN catalysts are used for propylene polymerization, the catalysts contain an internal donor. In further instances, the ZN catalysts are used with an external donor to obtain higher isotacticity. Examples of internal donors are esters of phthalic acid, including diisobutylphthalate. In some instances, phthalates are used as internal donors in combination with alkylalkoxysilanes as external donors. It is desirable to increase the intrinsic capability of the solid catalyst components to produce stereoregular polymers, thereby allowing the use of less stereoregulating internal or external donors.
It is also desirable to produce stereoregular polymers which maintain a high isotacticity while having high melt flow rate and a narrower molecular weight distribution.
It is also desirable to provide a catalyst system with improved stereospecificity while maintaining a high hydrogen response.